Fault-Driven Scan Chain Configuration For Test-Per-Clock

ABSTRACT

Aspects of the invention relate to using fault-driven techniques to generate scan chain configurations for test-per-clock. A plurality of test cubes that detect a plurality of faults are first generated. Scan chains for loading specified bits of the test cubes are then assigned to a stimuli group. From the plurality of test cubes, a test cube that detects a large number of faults that do not propagate exclusively to scan chains in the stimuli group is selected. One or more scan chains that are not in the stimuli group and are needed for observing the large number of faults are assigned to a compacting group. The number of scan chains either in the compacting group or in both of the compacting group and the stimuli group may be limited to a predetermined number.

FIELD OF THE INVENTION

The present invention relates to the field of circuit testingtechnology. Various implementations of the invention may be particularlyuseful for scan-based testing.

BACKGROUND OF THE INVENTION

Since its introduction in the late 1960′s, scan-based testing has gaineda wide acceptance as a structured design-for-test (DFT) methodology.This methodology connects memory elements such as flip-flops and latchesin a circuit to form scan chains and uses them to make internal nodes ofthe circuit highly controllable and observable. The controllability andobservability enables high-quality automated test generation for largeindustrial circuit designs.

A commonly used scheme of the scan-based testing is test-per-scan. In atest-per-scan system, a test pattern is first shifted into scan chainsand subsequently applied to the circuit-under-test. The test responsesgenerated by the circuit-under-test are then captured by and shifted outof the scan chains for analysis. In this testing scheme, the shifting inand out operations require much more clock cycles to perform than theactual testing (i.e. launching and capturing) operation does. Moreover,in part due to power concerns, the shifting clock is usually kept slowerthan the clock for the circuit's normal operation. This furtherincreases time for the data loading and unloading operations.

Consider, for example, a circuit design with 100,000 scan cells. Thesescan cells are divided into 500 scan chains, each 200 scan cells long.Assume the shifting and the normal operating clock frequencies of 50 MHzand 500 MHz, respectively. Applying 20,000 double-capture test patternsrequires 4,000,000 shift cycles at 50 MHz and 40,000 capture cycles at500 MHz. As a result, as low as 1% of cycles, or just 0.1% of time, isspent on the actual testing operation—applying test data and capturingtest response data. If a BIST (built-in self-test) method is used forthe same circuit design, the test time efficiency could be even lower.With 100K single-capture test patterns, 20,000,000 cycles are needed forscan shifting while only 100,000 cycles are needed for launching andcapturing. Using the same clock frequencies as the above example, 99.95%of test time is spent on scan shifting. The above two examplesdemonstrate that the test-per-scan scheme, though well developed andwidely adopted, is not very efficient with respect to testing time.

An alternative scheme, test-per-clock, has been developed mainly forBIST. In a conventional test-per-clock BIST system, the outputs of atest pattern generator are directly coupled to the inputs of thecircuit-under-test. Accordingly, a new test pattern is applied to thecircuit-under-test at every test clock cycle.

A 1979 paper by Konemann et al., “Built-in logic block observationtechniques,” 1979 IEEE Test Conference, which is incorporated herein byreference, describes such a system referred to as BILBO (built-in logicblock observer). A BILBO is composed of a flipflop register withadditional gates for shift and feedback operations. Four differentoperational modes can be performed by the BILBO: a mission mode (normalcircuit functional mode with scan cells working as latches), a linearshift register mode, a feedback mode, and a reset mode for registerresetting. In the feedback mode, the BILBO can work either as amultiple-input signature register (MISR) for compacting test responsesor as a linear feedback shift register (LFSR) for generatingpseudorandom test patterns. The latter is accomplished by keepingconstant values at the parallel inputs of the scan chain.

The 1979 paper describes an example of a test-per-clock architecturewith BILBOs working in pairs. One BILBO in a BILBO pair is configured tooperate in the feedback mode functioning as a linear feedback shiftregister. This LFSR BILBO generates and launches a test pattern everytest clock cycle. In the meantime, the other BILBO in the pair isconfigured to operate also in the feedback mode but functioning as amultiple-input signature generator. This MISR BILBO captures a testresponse every test clock cycle and compacts it with its previouslycompacted test response to form a new compacted test response. After anumber of test clock cycles, a test response signature is eventuallyshifted out of the MISR BILBO by switching its operational mode from thefeedback mode to the linear shift register mode. The unloaded testresponse signature can then be analyzed.

Another test-per-clock BIST system is described in a 1989 paper byKrasniewski et al., “Circular self-test path: a low cost BIST techniquefor VLSI circuits,” IEEE Trans. CAD, vol. 8, pp. 46-55, 1989, which isincorporated herein by reference. This system uses a feedback shiftregister with the last flipflop being supplied to the first flipflop.This shift register serves simultaneously for test pattern generationand test response compaction.

Compared to the test-per-scan scheme, the test-per-clock scheme is moretime-efficient because no slow shifting operation is needed for everytest pattern. However, the conventional test-per-clock BIST scheme mayhave a problem on power consumption. In the traditional test-per-clockBIST scheme, all scan chains change their contents every clock cycle nomatter whether they are used for pattern generation or for test responsecompaction. This can lead to excessive circuit toggling and thus powerdissipation. Moreover, the traditional test-per-clock scheme has beendeveloped mainly for BIST. It is desirable to develop a newtest-per-clock scheme that not only is more time-efficient than thetest-per-scan scheme but also addresses the limitations of the existingtest-per-clock scheme. Disclosed below are techniques related to atest-per-clock scheme based on dynamically-partitioned reconfigurablescan chains. More specifically for this disclosure, fault-driventechniques for identifying scan chains for various scan chainconfigurations are discussed.

BRIEF SUMMARY OF THE INVENTION

Aspects of the invention relate to using fault-driven techniques togenerate scan chain configurations for test-per-clock. With variousimplementations of the invention, a plurality of test cubes that detecta plurality of faults are first generated. Scan chains for loadingspecified bits of the test cubes are then assigned to a stimuli group.From the plurality of test cubes, a test cube that detects a largenumber of faults that do not propagate exclusively to scan chains in thestimuli group is selected. One or more scan chains that are not in thestimuli group and are needed for observing the large number of faultsare assigned to a compacting group. The number of scan chains either inthe compacting group or in both of the compacting group and the stimuligroup may be limited to a predetermined number. An analysis is thenperformed to determine a group of faults that can be detected by usingscan chains in the stimuli group for loading specified bits and scanchains in the compacting group for observing faults.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a test architecture for test-per-clockbased on dynamically partitioned and reconfigurable scan chains that maybe employed by various embodiments of the invention.

FIG. 2 illustrates a flow chart describing methods of test-per-clockbased on dynamically-partitioned configurable scan chains that may beemployed by various embodiments of the invention.

FIG. 3 illustrates an example of how the scan chains 121-125 in FIG. 1interact with each other during testing.

FIG. 4 illustrates a flow chart describing methods of using fault-driventechniques to generate scan chain configurations that may be employed byvarious embodiments of the invention.

FIG. 5 illustrates a programmable computer system with which variousembodiments of the invention may be employed.

DETAILED DESCRIPTION OF THE INVENTION

Various aspects of the present invention relate to using fault-driventechniques to generate scan chain configurations for test-per-clock. Inthe following description, numerous details are set forth for thepurpose of explanation. However, one of ordinary skill in the art willrealize that the invention may be practiced without the use of thesespecific details. In other instances, well-known features have not beendescribed in details to avoid obscuring the present invention.

Some of the techniques described herein can be implemented in softwareinstructions stored on a computer-readable medium, software instructionsexecuted on a computer, or some combination of both. Some of thedisclosed techniques, for example, can be implemented as part of anelectronic design automation (EDA) tool. Such methods can be executed ona single computer or on networked computers.

The detailed description of a method or a device sometimes uses termslike “generate,” “select” and “assign” to describe the disclosed methodor the device function/structure. Such terms are high-levelabstractions. The actual operations or functions/structures thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art. It should also be appreciated by one of ordinary skill in theart that the term “coupled” means “connected directly or indirectly.”

Although the operations of the disclosed methods are described in aparticular sequential order for convenient presentation, it should beunderstood that this manner of description encompasses rearrangements,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the disclosed flow charts and block diagrams typically donot show the various ways in which particular methods can be used inconjunction with other methods.

FIG. 1 illustrates an example of a test architecture for test-per-clockthat may be employed by various embodiments of the invention. The testarchitecture 100 comprises a test stimuli source 110, scan chains121-125, a test response collector 130, a configuration register 140 anda configuration source 150. The test stimuli source 110 may be an ATE(automated test equipment), a test data decompressor driven by an ATE, aPRPG (pseudorandom pattern generator), or any device that can supplytest patterns in some form. The ATE is commonly used for deterministictesting. The deterministic testing usually employs test patternsgenerated by ATPG (automatic test pattern generation). To test a largecircuit, compressed test patterns are often used. Before being shiftedinto scan chains, the compressed test patterns are decompressed by thetest data decompressor. For non-deterministic testing such as BIST, thePRPG can serve as the test stimuli source.

The test response collector 130 in the test architecture 100 may be thesame ATE used as the test stimuli source 110, a test response compactor,or any device that can collect and perhaps analyze test responsesshifted out of the scan chains. The test response compactor may compacttest responses spatially, temporally, or both. The spatial compactionmay be achieved by using a spatial compactor constructed with elementarygates such as AND, OR, NAND and NOR gates. The temporal compaction maybe accomplished by using, for example, a multiple-input signatureregister (MISR).

The test stimuli source 110 and the test response collector 130described above are similar to those used in a conventional testarchitecture. The scan chains 121-125 in the test architecture 100,however, are configured differently from conventional scan chains.Unlike conventional scan chains, scan chains used in various embodimentsof the invention can operate, depending on a control signal, in one ofat least three modes: a shifting-launching mode, acapturing-compacting-shifting mode and a mission mode. In the missionmode, scan chains perform regular circuit functions; in theshifting-launching and capturing-compacting-shifting modes, scan chainsare used for testing the circuit. Specifically, scan chains in theshifting-launching mode are responsible for controlling the internalstates of the circuit-under-test. They shift test data in and applyingthem to the circuit-under-test every clock cycle. Moreover, they do notcapture any test responses. Scan chains in thecapturing-compacting-shifting mode, on the other hand, are responsiblefor collecting test response data generated by the circuit-under-test.They shift out one bit of a previously compacted test response signalwhile compacting remaining bits of the previously compacted testresponse signal with a currently-captured test response signal to form acurrently compacted test response signal.

In FIG. 1, the scan chains 121, 123 and 125 are shown to work in theshifting-launching mode, the capturing-compacting-shifting mode and themission mode, respectively. The control signal for mode switching isgenerated by the configuration source 150 and applied to the scan chains121, 123 and 125 by the configuration register 140. FIG. 1 onlyillustrates one configuration (partition) of the scan chains. Differentpartitions of the scan chains can be obtained by varying the controlsignal. The content of the configuration register 140 can be reloadedduring a test session, depending on various requirements. In particular,the control signal may be varied with test patterns applied. Theconfiguration source 150 may be an on-chip device or an external device.

As seen in the figure, a majority of the scan chains operate in themission mode. This arrangement can alleviate power issues because logicstates associated with these scan chains closely resemble those when thecircuit works in its designed functional mode. Only a small portion ofthe scan chains may cause extrinsic circuit toggling. In a conventionalat-speed scan test, by contrast, a capture clock burst is applied to allscan chains which can result in a sudden current change within a fewnanoseconds and thereby circuit failures.

FIG. 2 illustrates a flow chart 200 describing methods of test-per-clockbased on dynamically-partitioned configurable scan chains that may beemployed by various embodiments of the invention. FIG. 3 illustrates anexample of how the scan chains 121-125 in FIG. 1 interact with eachother during testing. To simplify the figure, only one scan chain isused to represent scan chains in each operation mode. For ease ofunderstanding, methods of test-per-clock based on dynamicallypartitioned and reconfigurable scan chains that may be employedaccording to various embodiments of the invention will be described withreference to the flow chart 200 in FIG. 2 and the test architecture 100illustrated in FIG. 3. It should be appreciated, however, that alternateimplementations of a test architecture may be used to perform themethods of test-per-clock based on dynamically-partitioned configurablescan chains illustrated by the flow chart 200 according to variousembodiments of the invention. Likewise, the test architecture 100 may beemployed to perform other methods of test-per-clock baseddynamically-partitioned configurable scan chains according to variousembodiments of the invention.

In operation 210, test stimuli are shifted from a stimuli source (teststimuli source 110) into a first portion of a plurality of scan chainsin a circuit (scan chains 121) one bit per scan chain to form a new testpattern as illustrated by an arrow 350. The connections between the teststimuli source 110 and the scan chains 123 and 125 are blocked asillustrated by arrows 370 with broken lines.

Immediately after being formed, in operation 220, the new test patternis applied to the circuit. The new test pattern propagates through thecombinational part of the circuit until a test response reaches a secondportion of the plurality of scan chains (scan chains 123) as illustratedby an arrow 310. A circuit response caused by the new test pattern alsoreaches a third portion of the plurality of scan chains (scan chains125) as illustrated by an arrow 320. This circuit response willcirculate within the circuit and eventually reach the scan chains 123 asillustrated by arrows 330.

In operation 230, the scan chains 123 shift out previously compactedtest response data one bit per scan chain to the test response collector130 as illustrated by an arrow 360. The connections between the testresponse collector 130 and the scan chains 121 and 125 are blocked asillustrated by arrows 380 with broken lines.

At about the same time as the operation 230, in operation 240, the testresponse corresponding to the new test pattern is compacted with thepreviously compacted test response data to generate newly compacted testresponse data in the scan chains 123. As shown in FIG. 3, the scanchains 123 are also employed to drive the circuit as illustrated byarrows 340, which is referred to as a mode ofcapturing-compacting-shifting without blocking. This functionality canbe disabled if needed and the scan chains operate in a mode referred toas a mode of capturing-compacting-shifting with blocking.

As noted previously, the scan chains 121, 123 and 125 are configured tooperate in the shifting-launching mode, thecapturing-compacting-shifting mode and the mission mode, respectively,based on the control signal stored in the configuration register 140.The above four operations may be performed once for every clock cyclefor a predetermined number of times. At the end, a different controlsignal may be loaded into the configuration register 140 to reconfigurethe scan chains. The reconfigured scan chains are then employed for thenext test pattern(s). This dynamic partitioning and reconfiguringapproach contrasts not only the conventional scan method but also theBILBO approach discussed in the background section.

Another difference between various embodiments of the present inventionand the BILBO-based techniques may lie in the operation of test responsecompaction. As previously noted, a BILBO scan chain in the feedback modeworks as a multiple-input signature generator for compacting testresponses: Test responses are captured and compacted for a number ofclock cycles (corresponding to the same number of test patterns) togenerate a signature. No bit of the compacted test response signal isshifted out during that time period. Instead, the signature is shiftedout after a number of test patterns are applied. By contrast, a scanchain in the capturing-compacting-shifting mode according to variousembodiments of the invention does not have a feedback loop. Moreover,one bit of previously compacted test response data is shifted out everyclock cycle.

By adopting the test-per-clock scheme, various embodiments of theinvention remove the lengthy scan shift-in phase used in thetest-per-scan scheme and perform launch-capture testing every clockcycle. This allows more test patterns to be applied within a certainperiod of time and may improve the fault coverage. Alternatively, onecan choose to apply the same number of test patterns as that of aconventional scan test, yet in a much shorter period of time, therebyreducing the test cost.

By dynamically partitioning and reconfiguring the scan chains, thedisclosed test-per-clock scheme can allow the majority of scan chains tooperate in the mission mode to alleviate power issues withoutsignificant impact on test generation and fault coverage. The controlsignal remains static after a given configuration is established. Itcan, therefore, be placed and routed with no rigid timing constrainssimilar to those of scan enable signals whose distribution and delivery,especially for the at-speed test purpose, must meet non-flexible timingclosure conditions. The low-power capabilities may enable applying testpatterns at higher, close to the functional, frequency, which canfurther increase fault coverage metrics.

To realize low power and high test quality potentials of the disclosedtest-per-clock scheme, scan chain configurations need to be carefullydesigned. Disclosed below are fault-driven techniques for designing scanchain configurations.

FIG. 4 illustrates a flow chart 400 describing methods of usingfault-driven techniques to generate scan chain configurations that maybe employed by various embodiments of the invention. The flowchartstarts with operation 410, in which a plurality of test cubes thatdetect a plurality of faults are generated. This operation may beperformed by a conventional automatic test pattern generator. In anotherembodiment of the invention, an automatic test pattern generator may beguided in such a way that its forward and backward propagation as wellas justification steps select completely disjoint subsets of scan chainsdeployed to load specified bits and to observe faults. During faultsimulation, the automatic test pattern generator records detected faultsand their propagation sites (observation sites) for each of the testcubes.

Next, in operation 420, scan chains for loading specified bits of thetest cubes are assigned to a stimuli group. Scan chains in the stimuligroup are to be configured to operate in the shifting-launching modeunder a particular scan chain configuration. Observation sites belongingto scan chains in the stimuli group are removed from the observationsites for each of the test cubes because test patterns are shifted intothese scan chains every clock cycle and these scan chains are not usedfor capturing test responses in the disclosed test-per-clock scheme.

In operation 430, a test cube that detects a large number of faults thatdo not propagate exclusively to scan chains in the stimuli group isselected from the plurality of test cubes. In some embodiments of theinvention, the test cube is the one that detects the largest number offaults that do not propagate exclusively to scan chains in the stimuligroup.

In operation 440, one or more scan chains that are not in the stimuligroup and are needed for observing the large number of faults areassigned to a compacting group. Scan chains in the compacting group areto be configured to operate in the capturing-compacting-shifting modeunder the scan chain configuration associated with the stimuli group.The capturing-compacting-shifting mode may be a mode ofcapturing-compacting-shifting with blocking or without blocking Theoperation 440 may also limit the number of scan chains either in thecompacting group or in both of the compacting group and the stimuligroup to a predetermined number. This constraint helps keep a majorityof the scan chains in the mission mode.

In operation 450, an analysis is performed to determine a group offaults that can be detected by using scan chains in the stimuli groupfor loading specified bits and scan chains in the compacting group forobserving faults. One of the analyses may be the SandiaControllability/Observability Analysis (SCOAP) analysis, a well-knownmethod. The determined group of faults may be removed from the pluralityof faults.

Various embodiments of the invention may be implemented through theexecution of software instructions by a computing device, such as aprogrammable computer. FIG. 5 shows an illustrative example of such aprogrammable computer (a computing device 501). As seen in this figure,the computing device 501 includes a computing unit 503 with a processingunit 505 and a system memory 507. The processing unit 505 may be anytype of programmable electronic device for executing softwareinstructions, but will conventionally be a microprocessor. The systemmemory 507 may include both a read-only memory (ROM) 509 and a randomaccess memory (RAM) 511. As will be appreciated by those of ordinaryskill in the art, both the read-only memory (ROM) 509 and the randomaccess memory (RAM) 511 may store software instructions for execution bythe processing unit 505.

The processing unit 505 and the system memory 507 are connected, eitherdirectly or indirectly, through a bus 513 or alternate communicationstructure, to one or more peripheral devices. For example, theprocessing unit 505 or the system memory 507 may be directly orindirectly connected to one or more additional memory storage devices,such as a “hard” magnetic disk drive 515, a removable magnetic diskdrive 517, an optical disk drive 519, or a flash memory card 521. Theprocessing unit 505 and the system memory 507 also may be directly orindirectly connected to one or more input devices 523 and one or moreoutput devices 525. The input devices 523 may include, for example, akeyboard, a pointing device (such as a mouse, touchpad, stylus,trackball, or joystick), a scanner, a camera, and a microphone. Theoutput devices 525 may include, for example, a monitor display, aprinter and speakers. With various examples of the computer 501, one ormore of the peripheral devices 515-525 may be internally housed with thecomputing unit 503. Alternately, one or more of the peripheral devices515-525 may be external to the housing for the computing unit 503 andconnected to the bus 513 through, for example, a Universal Serial Bus(USB) connection.

With some implementations, the computing unit 503 may be directly orindirectly connected to one or more network interfaces 527 forcommunicating with other devices making up a network. The networkinterface 527 translates data and control signals from the computingunit 503 into network messages according to one or more communicationprotocols, such as the transmission control protocol (TCP) and theInternet protocol (IP). Also, the interface 527 may employ any suitableconnection agent (or combination of agents) for connecting to a network,including, for example, a wireless transceiver, a modem, or an Ethernetconnection. Such network interfaces and protocols are well known in theart, and thus will not be discussed here in more detail.

It should be appreciated that the computer 501 is illustrated as anexample only, and it not intended to be limiting. Various embodiments ofthe invention may be implemented using one or more computing devicesthat include the components of the computer 501 illustrated in FIG. 5,which include only a subset of the components illustrated in FIG. 5, orwhich include an alternate combination of components, includingcomponents that are not shown in FIG. 5. For example, variousembodiments of the invention may be implemented using a multi-processorcomputer, a plurality of single and/or multiprocessor computers arrangedinto a network, or some combination of both.

Some other embodiments of the invention may be implemented by softwareinstructions, stored on a non-transitory computer-readable medium, forinstructing one or more programmable computers/computer systems toperform operations such as those shown in the flow chart 400 in FIG. 4.As used herein, the term “non-transitory computer-readable medium”refers to computer-readable medium that are capable of storing data forfuture retrieval, and not propagating electro-magnetic waves. Thenon-transitory computer-readable medium may be, for example, a magneticstorage device, an optical storage device, a “punched” surface typedevice, or a solid state storage device.

Conclusion

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques that fallwithin the spirit and scope of the invention as set forth in theappended claims. For example, while specific terminology has beenemployed above to refer to electronic design automation processes, itshould be appreciated that various examples of the invention may beimplemented using any desired combination of electronic designautomation processes.

What is claimed is:
 1. A method, executed by at least one processor of acomputer, comprising: generating a plurality of test cubes that detect aplurality of faults; assigning scan chains for loading specified bits ofthe test cubes to a stimuli group; selecting, from the plurality of testcubes, a test cube that detects a large number of faults that do notpropagate exclusively to scan chains in the stimuli group; and assigningone or more scan chains that are not in the stimuli group and are neededfor observing the large number of faults to a compacting group.
 2. Themethod recited in claim 1, further comprising: performing an analysis todetermine a group of faults that can be detected by using scan chains inthe stimuli group for loading specified bits and scan chains in thecompacting group for observing faults.
 3. The method recited in claim 2,wherein the analysis is a SCOAP analysis.
 4. The method recited in claim1, wherein the assigning one or more scan chains is performed under apredefined condition with respect to number of scan chains in thecompacting group or the total number of scan chains in the stimuli groupand the compacting group.
 5. The integrated circuit recited in claim 1,wherein the selected test cube is a test cube that detects the largestnumber of faults that do not propagate exclusively to scan chains in thestimuli group.
 6. The integrated circuit recited in claim 1, whereinscan chains in the compacting group are to be configured to operate in acapturing-compacting-shifting mode and scan chains in the stimuli groupare to be configured to operate in a shifting-launching mode.
 7. Theintegrated circuit recited in claim 6, wherein thecapturing-compacting-shifting mode is a mode ofcapturing-compacting-shifting with blocking
 8. The integrated circuitrecited in claim 6, wherein the compaction mode is a mode ofcapturing-compacting-shifting without blocking
 9. One or morenon-transitory computer-readable media storing computer-executableinstructions for causing one or more processors to perform a method, themethod comprising: generating a plurality of test cubes that detect aplurality of faults; assigning scan chains for loading specified bits ofthe test cubes to a stimuli group; selecting, from the plurality of testcubes, a test cube that detects a large number of faults that do notpropagate exclusively to scan chains in the stimuli group; and assigningone or more scan chains that are not in the stimuli group and are neededfor observing the large number of faults to a compacting group.
 10. Theone or more non-transitory computer-readable media recited in claim 9,wherein the method further comprises: performing an analysis todetermine a group of faults that can be detected by using scan chains inthe stimuli group for loading specified bits and scan chains in thecompacting group for observing faults.
 11. The one or morenon-transitory computer-readable media recited in claim 10, wherein theanalysis is a SCOAP analysis.
 12. The one or more non-transitorycomputer-readable media recited in claim 9, wherein the assigning one ormore scan chains is performed under a predefined condition with respectto number of scan chains in the compacting group or the total number ofscan chains in the stimuli group and the compacting group.
 13. The oneor more non-transitory computer-readable media recited in claim 9,wherein the selected test cube is a test cube that detects the largestnumber of faults that do not propagate exclusively to scan chains in thestimuli group.
 14. The one or more non-transitory computer-readablemedia recited in claim 9, wherein scan chains in the compacting groupare to be configured to operate in a capturing-compacting-shifting modeand scan chains in the stimuli group are to be configured to operate ina shifting-launching mode.
 15. The one or more non-transitorycomputer-readable media recited in claim 14, wherein thecapturing-compacting-shifting mode is a mode ofcapturing-compacting-shifting with blocking.
 16. The one or morenon-transitory computer-readable media recited in claim 14, wherein thecompaction mode is a mode of capturing-compacting-shifting withoutblocking.